Status ofMoeller Polarimeter

Peter OtteApril 7, 200811th Collaboration Meeting, Dubrovnik

Why built a Moeller Polarimeter?

Aim: measuring electron-beam polarization

Using Moeller scattering

What is Moeller scattering? (1/2)

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Rk

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CMCM

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d

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1

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Spin component from beam and radiator

involved: 2 electrons(Composed by 3 amplitudes in 1st order QED)

cross section:

e-

e-

tensor gives analysis powerMoeller c.s. for unpol.

What is Moeller Scattering? (2/2)

Key features of this two-electron process: huge energy transfer, T~1/m energy sum of both electrons sums up to beam energy:

Angular distributionof scattering angle:

BeamEEE 21

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n fr

om „

Dip

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it Le

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Bremsstrahlung

Moeller scattering°

Tagging system

NN

NNA

BeamEEE 21 setup:

finding several pairs of ladder channels with a sum of EBeam

→ coincidental detection of both Moeller electrons

measurement:

(arrows indicate spin alignment of beam and target)

Background processes

Processes that produce the same footprint on the ladder:

Moeller scattering always:

E1+E2 = EBeam

time correlated

Bremsstrahlung accidental coincidences not time correlated

Pair production(3 body process.)

sometimes: E1 + E2 = EBeam

time correlated

How can we reduce triggers generated by bremsstrahlung and pair production events during our measurement to receive better statistics?

Better results by…

forcing the right energy sum→ this becomes the trigger condition

forcing correlation in time→ done in offline analysis usingtime-spectrum

attaching additional detectors

Erik Heid 2006

Rom

an L

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Beam profile measurements

…

16 fibre detectors

View towards tagger ladder:

Beam profile measurements behind the tagger with primary beam

detector collected electrons with approx. 400 MeV Result: FWHM of 8 / 9 mm in x- and y-direction

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0

0

200

400

600

800 measurement fit of exp. data geant simulation

Experiment 16/2/2008

cou

nts

horizontal position in cm towards Beam Dumptowards high energy photons

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5

0

200

400

600

800

2 measurements measuremt, reference @ 0cm measuremt, reference @ -1,6cm fit of exp. data Geant simulation

cou

nts

vertical position in cmtowards basement towards ceiling

Experiment 16/2/2008

horizontal vertical

8mm 9mm

vertical profile measurement behind the taggerwith radiator

(Planned: Simulation with radiator)

-2 -1 0 1 2 3 4 5 6 7 80

100

200

300

400

500 measurement between: -2 and 4 cm 0 and 6 cm 2 and 8 cm Fit of data around peak

a

rb. r

ate

/Hz

vertical position in cmtowards basement towards ceiling

FWHM:2.5cm

Why a new Moeller Polarimeter?

why demanding a new one? new gap width higher rate: aiming for 108 tagged

photons per sec ability to adapt the trigger logic to

tagger calibration more easily

The new trigger board:

(Uppsala board)

faster

smaller

more flexible

Status so far

Logic board ready programmed First tests in beam next week

additional detectors (out of plane) necessary?

appendix

Comparison with old Moeller Polarimeterwhy demanding a new one? new gap width higher rate: aiming for 108 tagged photons per sec ability to adapt the trigger logic to tagger calibration

more easily

old (~1995) new (2008)

Trigger in real-time yes yes

Reconfigure the logic

by wire by software

Coincidences 3 pairs à 16

latter channels

between arbitrary latter channels, not only pairs

up to 256 channels can be handled

Decision time ? faster: 20ns, less cables and electronic

Size big: standard electronic

small: one sheet of paper and power consumption of about 4 Watts